1. Technical Field
The invention relates to a system for ventilating a pair of juxtaposed blade platforms in a turbomachine.
Ventilating systems found in turbomachines generally involve blowing a stream of gas onto a portion of a structure which is at a temperature different than the gas in order to protect the structure from excessive heating or, in certain applications, in order to regulate its thermal expansion so as to control a clearance and thereby improve the throughput of the machine.
The invention concerns the ventilation of blade platforms installed side by side in the same circumferential array of blades so as to cover the cavities separating the lower portions of the blades and to give the flow path of the machine a more uniform contour. The cavities can be put to use by blowing the ventilating gas into them. However, the ventilating gas would leave the cavities via gaps between the platforms and would therefore have little effect.
2. Summary of the Prior Art
It is known to place sealing means under the platforms to block these gaps between the platforms. The sealing pieces are designed to allow the ventilating gas to reach the portions of the platforms that it is desired to ventilate, and these portions are equipped with vent orifices that the gas passes through in order to leave the cavities. The desired heat exchange is achieved principally by virtue of the gas passing through these vent orifices.
One such system is described in French Patent 2758855, wherein the sealing pieces are metal sleeves, formed of sheet metal, which rest under the underside of the platforms, centrifugal forces produced as the blades rotate urging the sleeves against the platforms. The sheet metal sleeves have apertures through which the ventilating gas passes in order to reach the platforms, these apertures opening into collecting channels cat in the platforms. These channels are at the ends of serpentine coils, that is to say sinuous ducts, through which the ventilating gases must travel in order to reach the vent orifices passing through the platforms. Heat exchange is by convection. However, it has been concluded that this arrangement is not effective enough.
The object of the invention is therefore to improve the sheet metal sealing sleeves so as to increase the heat exchange attributable to the ventilating gas without compromising sealing, while at the same time obtaining satisfactory damping or the blade platform vibration.
Accordingly, the invention provides a system for ventilating a pair of juxtaposed blade platforms covering a cavity in a turbomachine, comprising vent orifices passing through the platforms, a sealing sleeve having apertures passing through it arranged under the platforms in the cavity, and means for blowing ventilating gas into the cavity, wherein said sleeve is provided with bosses at the sites of said apertures whereby chambers are defined between said platforms and said bosses, said vent orifices opening into said chambers.
This means that the ventilating gas passing through the apertures in the sleeve is blown toward the platforms at a speed which increases heat exchange by virtue of the impact of the gas against the platforms.
Advantageously, the bosses extend parallel to a junction between the platforms so that each encompasses a number of apertures or a number of vent orifices. It is also advantageous for each of the vent orifices to face a respective one of the apertures, so as to make it easier for the ventilating gases to flow.